Many observers have noted that the crossing of different animal species produces an offspring one sex of which is rare or absent, or if present sterile, whilst occasionally the missing sex is represented by intermediate forms. Doncaster(1) conclnded that the missing sex was generally thc female, bnt, as will be shown later, this is by no means always the case. I believe, however, that the following rule applies to all cases so far observed, with one certain, and a few donbtfid exceptions :-When in the F~ old'spring of two dil~'erent animal races one sex is absen}, rare, or sterile, that sex is the heterozygous sex. Journ. of Gen. xIz 7 ]Kother Father Offspring Cerura ermi.nea ... Cerura vi.nu.la ......

New factors arise in a species by the process of mutation. The frequency of mutation is generally small, but it seems probable that it can sometimes be increased by changes in the environment (1,2). On the whole mutants recessive to the normal type occur more commonly than dominants. The frequency of a given type of mutation varies, but for some factors in Drosophila it must be less than 10−6, and is much less in some human cases. We shall first consider initial conditions, when only a few of the new type exist as the result of a single mutation; and then the course of events in a population where the new factor is present in such numbers as to be in no danger of extinction by mere bad luck. In the first section the treatment of Fisher (3) is followed.

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